In mice the lack of an easily administered and selective test for vestibular function is a significant impediment for studies of the vestibular system, especially for those studies that seek to assess quantitatively functional losses associated with aging or the efficacy of drug treatments or genetic manipulations. The overall goal of this proposal is to develop and validate easily used functional vestibular tests for mice to use in such studies. There are 2 specific aims. The first aim is to optimize measurements of the vestibulo-collic reflex (VCR) in animals subjected to en bloc dynamic rotation about earth vertical and earth horizontal axes. Preliminary data suggests the feasibility of this approach, but the extent of the vestibular system's contribution to the response must still be determined. Since mice orient their heads with respect to gravity, we will also develop a, simplified method of measuring head orientation based on an analysis of video recordings of head position during static tilts. These tests are based on the hypothesis that the neck reflexes are dependent on vestibular sensory inflow and act to stabilize head position and orientation during whole body motion in space. Sensory cell losses or peripheral innervation changes should result in measurable changes in vestibular neck reflexes. Aim 2 will assess changes in the static and dynamic VCR in a cohort of aging mice by comparing changes in VCR with changes in peripheral morphology or synaptic organization. C57BL/6 mice aged 12,18, 24 and 30 months will be tested to determine if the efficacy of vestibular neck reflexes declines with age. The vestibular test data will be compared to non-specific methods of assessing vestibular and motor function such as "balance beam and rota-rod tests. The temporal bones of the aged mice will be subjected to morphological analyses to determine if there are age related changes in the numbers of sensory hair cells or hair cells with calyx endings (Type I hair cells).